A tweet by Battlefield Bad Company 2 lead designer David Goldfarb that's retweeted by the BFBC2 feed says that even if you stand on your head, they are not adding the ability to go prone in the military shooter sequel: "Fyi we will not add prone based on anything. You can stop asking now. :)"

What others have said here are correct, there is plenty of cover in the game to compensate for not having prone. I've been sniped many times from bushes and from behind cover already. This is a non-issue for the most part.

There is a thread in the BC2 beta EA forum and at least every other post in that thread is asking for prone. DICE/Goldfarb are sticking to their guns on this one. Pun intended.